1. Field of the Invention
The present invention relates generally to pressure vessels of the type used to house a water filter. More particularly, the invention is directed to a two-section pressure vessel housing designed to operate at full household water pressure wherein a threaded collar is provided to secure the two housing sections together.
2. Description of Related Art
Pressure vessels of the type used in domestic water purifying systems are generally of a molded plastic construction, generally cylindrical in shape and are adapted to house a generally cylindrical filter element. These pressure vessels normally comprise two sections, a bottom cap section and a major housing section, threadably secured together and sealed with an elastomeric seal. It is preferably to mold a pressure vessel with as few seams as possible. Thus, the bottom section is molded with external threads. The major housing section is molded without threads but is adapted to receive a molded internal thread ring which is welded therein in a subsequent assembly step. When pressure vessels of this type are used in a domestic water purifying apparatus, they normally sit in an upright position. The bottom section is provided with a molded substantially flat base in order to make it less difficult for the unit to stand upright. In one assembly step, a filter element is disposed within the housing and threadably or otherwise secured to the bottom section of the housing. The filter has a fluid outlet centered over an outlet port which is in fluid communication therewith. An inlet port is disposed adjacent the outlet port and is also in fluid communication with the filter. In yet another assembly step, molded elbow fittings are welded to both the inlet port and the outlet port. The assembly steps required to weld the internal thread ring and the elbow fitments to the housing adds a substantial cost per unit produced.
In operation, the inlet port is plumbed to a pressurized source of water while the outlet port is plumbed to the terminus of the discharge valve of a faucet. Thus, the pressure vessel remains under constant household water pressure up to 125 PSIG and pressure peaks which may be higher. When the discharge valve of the faucet is opened, water which has been filtered through the filter element flows from the filter through the outlet port and out the faucet thereby causing a substantial pressure drop within the pressure vessel. When the faucet valve is closed, full household water pressure resumes within the pressure vessel. Over time, cycling between low pressure and high pressure within the vessel amplifies the effect of any weakness in the vessel construction. The weak areas manifest over a period of repeated use mainly at the weld points where the internally threaded molded ring is welded to the inside of the housing and the molded elbow fitments to the inlet and outlet ports. Eventual fatiguing of these points brings forth both water leakage and the cracking and breaking of the molded components of the pressure vessel. Further, since one section of the housing, namely the bottom section, comprises a right circular cross section at the base, stress concentrations appear at the point where the bottom section is threaded into the top section. Repeated removal and replacement of this bottom section during regular maintenance intervals raises the potential for eventual breakdown of materials caused by stress and fatigue from repeated use.
In order to provide a more aesthetic appeal, plastic domestic water purifier housings are molded using various proportions of pigmented polymer beads to give the molded plastic housing a desired color. The housings are generally molded in a frusto-conical shape which not only provides aesthetic appeal, but makes the molding process less difficult. However, pigmented plastics are generally inherently weaker than nonpigmented plastics, because in order to achieve the desired pigment, various proportions of pigment polymers must be added to the molding resin thereby diluting the strength characteristics of the pure resin. Further, frusto-conical pressure vessel housings, by virtue of their geometry, possess inherent stress concentrations posing a significant potential for weakness in the housing itself and eventual breakdown due to the repeated cycles of pressure fluctuation within the housing through use.
Domestic water filter systems periodically require maintenance resulting in the need to replace the filter element. In order for the consumer to access the filter element, the consumer must first shut off the water supply, use an assortment of plumbing tools to mechanically remove the inlet and outlet port plumbing connections, invert the entire housing and mechanically turn and unscrew the bottom portion of the housing away from the upper portion. While conventional plumbing tools are normally required to remove the inlet and outlet port connections, the bottom section is often provided with a diametric groove wherein an unconventional or specially designed tool often must be used to facilitate its removal.